1. Field of the Invention
The present invention relates to graphics machines, and more particularly to a method and apparatus for automatically transferring writing tools in graphics machines.
A particular field of application of the invention is pen plotters, however, the invention is applicable, more generally, to any graphics machine of the type comprising: a write head provided with means for fixing a removable writing tool; actuator means for displacing the writing tool as carried by the write head along a first direction between a working position in contact with a print medium and a rest position out of contact with the print medium; and means for displacing the write head along a second direction relative to the print medium in order to plot predetermined graphics patterns thereon by means of the writing tool carried by the head.
2. Description of Related Art
Graphics machines, and in particular X-Y pen plotters are known in which a write head is generally movable in translation along a single horizontal direction (Y) parallel to the print medium, at least in the working zone of the write head, while the print medium is engaged with drive means in order to enable it to be displaced in another horizontal direction (X) perpendicular to the first. A desired plot is produced on the print medium by an appropriate combination of movements in the X and Y directions of the print medium and of the write head while it is carrying a writing tool in the working position.
Graphics machines of the above type are known in which means are provided for enabling the writing tool to be changed automatically. Such a transfer is obtained by displacing the write head to a writing tool transfer position in the immediate vicinity of a storage device in which a plurality of writing tools are stored. The storage device is generally in the form of a rotary magazine or turret with stations formed in its periphery for receiving respective writing tools.
A device for changing the writing tool in a drawing machine is described in U.S. Pat. No. 4,683,476 (Ferrari et al). In this prior device, the write head is provided with means exerting a first magnetic attraction force on the body of the writing tool in order to hold the tool fixed to the head, and the storage device is provided with means exerting a second magnetic force for holding the writing tools which are stored in their stations. The intensity of at least one of the two magnetic attraction forces is capable of being varied to enable a writing tool to be transferred from the write head to the storage device and back again under the effect of the difference between the magnetic attraction forces acting on the tool. With this prior device, the writing tool is fixed to the write head or to the storage device under the effect of a magnetic force extending transversely relative to the tool-lowering and tool-raising directions. This provides a structure which is simpler than other prior art devices for changing writing tools in which the tool is released from the write head or from the storage device by unlocking mechanical systems under the effect of contact between the head and the storage device.
In the device of above-mentioned U.S. Pat. No. 4,683,476, a moving cap is associated with each station in the storage device for the purpose of protecting and enclosing the tip of the writing tool contained in the station. Means need to be provided for displacing the cap in order to enable the writing tool to be transferred in and out of the corresponding station. Cap displacement is obtained by a fixed magnet which exerts a magnetic attraction force on that one of the caps facing the write head in its transfer position.